It is usual practice to produce medium or low carbon ferromanganese by decarburizing molten high carbon ferromanganese in converters. It is known that the decarburization reaction rate of the molten high carbon ferromanganese is determined according to the following major factors.
1. Temperature of molten ferromanganese
2. Agitation of molten ferromanganese
3. Gas composition blown from bottom tuyeres.
Among these factors, the temperature of the molten ferromanganese is set out in detail in U.S. Pat. No. 3,305,353. According to this patent, the conditions of causing the decarburization to proceed quickly in a pure oxygen top-blown converter where the oxygen gas is blown on the surface of the molten-high carbon manganese, include a molten metal temperature not lower than 1550.degree. C. It is set out that in order to cause decarburization to proceed while suppressing oxidation of Mn in a medium or low carbon concentration region, the molten metal temperature should be not lower than 1700.degree. C. before the carbon concentration of the molten metal reaches 1.5 wt. % or below.
Among the above factors, the agitation of the molten ferromanganese is set forth in Japanese Patent Publication No. 57-27166. According to this publication, the decarburization proceeds very efficiently when using a converter which has double-pipe bottom tuyeres. More particularly, the yield of Mn amount in a pure oxygen top-blown converter (i.e. a converter which has no bottom tuyeres at the bottom thereof) is 79%. With a pure oxygen top blown converter having double-pipe bottom tuyeres, the yield of Mn amount is very high at 92%. The molten metal is more strongly stirred by the bottom-blown gas, thereby obtaining a high decarburization reaction efficiency.
The gas composition blown through the bottom tuyeres which is one of the factors is set out in Japanese Patent Publication No. 3-55538 in which reference is made to the above-indicated U.S. Pat. No. 3,305,352 and Japanese Patent Publication No. 57-27166 as prior art. According to this publication, when a bottom-blown gas is charged through the bottom tuyeres of a pure oxygen top blown converter, the composition of the bottom-blown gas is important. In the publication, argon gas, nitrogen gas, carbon dioxide gas or mixtures are used as the bottom-blown gas.
However, if argon gas, nitrogen gas, carbon dioxide gas or mixtures thereof are used on an industrial scale in decarburization of molten high carbon ferromanganese in a pure oxygen top-blown converter having bottom tuyeres, there arise the following problems.
(1) The use of nitrogen gas has the problem that the nitrogen concentration in ferromanganese increases. Usually, nitrogen is saturated at a concentration of 300-400 ppm in high carbon ferromanganese. As the decarburization proceeds, the nitrogen concentration increases. Especially, in medium or low carbon ferromanganese whose carbon concentration is 2.5% or below, the nitrogen concentration increases in proportion to the total of the bottom-blown nitrogen gas. Finally, the concentration increases up to a maximum value of approximately 10,000 ppm.
(2) The use of carbon dioxide gas has the problem that it considerably damages the bottom tuyeres and refractories therearound. The reason for this is that when carbon dioxide gas contacts with the high temperature molten metal just on the bottom tuyeres, the following dissociation reaction proceeds EQU CO.sub.2 (g).fwdarw.CO (g)+1/2O.sub.2 (g).
The released O.sub.2 (g) serves to damage refractories. As a result, the life of the converter refractories lowers, with a considerable increase in running costs.
As stated above, when medium or low ferromanganese is manufactured on an industrial scale using argon gas, nitrogen gas or carbon dioxide gas as the bottom-blown gas, the resultant medium or low carbon ferromanganese has a high nitrogen concentration thereby producing the problem that the bottom tuyeres and nearby refractories are damaged.
Under these circumstances in the art, the present invention has for its first object the provision of a method for manufacturing medium or low carbon ferromanganese whereby bottom tuyeres and nearby refractories are prevented from being damaged, with low running costs.
In order to employ argon gas, nitrogen gas or carbon dioxide gas on an industrial scale, a large-scale gas generator is necessary. In case where a commercially sold gas is used, large-scale storage and evaporation facilities are required. This will prevent the problem that production cost of the medium or low carbon ferromanganese becomes high.
Accordingly, the present invention has its second object the provision of method and apparatus for manufacturing meduium or carbon ferromanganese at low costs.